The relationship between the level of C-reactive protein (CRP) in the sera of mice and resistance to Staphylococcus aureus infection after the injection of Escherichia coli endotoxin was studied. The CRP level was essentially unchanged at 6 hr after endotoxin, and resistance was slightly decreased. At 24 hr after endotoxin, both CRP levels and resistance were increased. Since the increase in the CRP level and resistance appeared to be associated, it was of interest that, when mouse CRP was tested for in vitro reactions with several strains of bacteria, cells of all species of gram-positive bacteria tested (including S. aureus) were agglutinated by CRP. E. coli was not agglutinated under the conditions of the test. It is proposed that mouse CRP is an opsonin, and possibly a lysin, and is involved in nonspecific resistance to infection with S. aureus. Results of a recent study on staphylococcal arthritis in chickens suggested that the early production of C-reactive protein (CRP) is associated with resistance to Staphylococcus aureus (Patterson and Mora, 1965). The production of CRP in humans and Cx-reactive protein (CxRP) in rabbits can be induced by a variety of stimuli, including endotoxins (Hedlund, 1961). It is of interest, therefore, that many of these agents also can enhance resistance of mice to infection with S. aureus (Shilo and Wolman, 1958; Springer et al., 1961; Higginbotham, 1964) as well as with gram-negative bacteria (Rowley, 1964). Considerable attention has been given to the possible role of both cellular and humoral factors in the endotoxin-induced nonspecific resistance in mice (Rowley, 1964; Shilo, 1959). However, there appear to have been no studies relating CRP to resistance in this species. The present study was conducted to determine whether the endotoxininduced increase in resistance of mice to S. aureus infection was associated with the appearance of CRP-like substance in the sera of these animals. Evidence is presented that a substance that reacts with Cx-polysaccharide and antisera to human CRP was elevated in sera of mice during the early stage of increased resistance to infection. It was of interest that this protein caused the agglutination of cells of S. aureus as well as several other species of gram-positive bacteria.
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